The general basis of cancer is the loss of cell growth control mechanisms and the resulting abnormal proliferation of cells. Traditionally, a universal paradigm in oncogenesis is the accumulation of mutations in the coding or regulatory regions of cellular genes such as oncogenes and tumor suppressor genes. These mutations lead to perturbations of the normal cellular signaling processes that govern cellular proliferation and development. However, recent research has revealed a new class of Ranks termed non-coding Ranks (ncRNA) (also referred to as functional RNA, or frank). ncRNAs include a variety of RNA molecules including, but not limited to, miRNA (micron), rena (ribosomal RNA), siRNA (small interfering RNA), snRNA (small nuclear RNA), snmRNA (small non-mRNA), snoRNA (small nucleolar RNA) and stRNA (small temporal RNA). The functions of these ncRNAs are diverse and are still being determined. Many of the ncRNA molecules interact with proteins to form rib nucleoprotein (RNP) complexes.
miRNA has emerged as one of the more intriguing members of the ncRNA class. miRNA has been determined to be important for cellular growth, development and homeostasis and research points to the involvement of these miRNAs in a variety of disease states, such as cancer. miRNAs are short nucleotide transcripts cleaved from a larger hairpin precursor. In certain embodiments, the miRNA are 19-23 nucleotides in length. Research suggests that the Dicer protein and related proteins are involved in the cleavage of the RNA hairpin precursor to form the miRNAs (Hutvagner et al., Science 293: 834-838, 2001; Ketting et al., Gene & Development. 15: 2654-2659, 2001). Many miRNAs, often with highly conserved sequences, are present in the genomes of organisms, such as, but not limited to, Caenorhabditis elegans, Drosophila, rats, mice, and humans (Lagos-Quintata et al., Science 294: 853-858 2001; Lagos-Quintata et al., Curr Biol 12, 735-739, 2002; Lee and Ambros Science 294: 862-86 2001; Mourelatos et al. Gene & Development, 16: 720-7282002; Dostie et al. RNA 9: 180-186, 2003). In some instances, the miRNAs are organized in the genome as clusters, sometimes separated by intervals as short as a few nucleotides.
The roles proposed for miRNAs are diverse. miRNAs are postulated to be involved in regulation of mRNA stability and translation, heterochromatin formation, genome rearrangement, and DNA excision (Baulcombe Science 297:2002-2003, 2002). In C. elegans, miRNAs coordinate the translation of heterochromic genes (Banerjee et al., BioEssays, 24: 119-129, 2002). Two C. elegans miRNAs, lin-4 and let-7, control developmental timing by forming imperfect base pairing with elements within the 3′ UTR of target mRNAs and attenuating their translation (Lee et al., Cell 75:843-854, 1993; Wightman et al., Cell. 75(5):855-62, 1993). A specific miRNA in Arabidopsis is known to direct the cleavage of transcripts encoding several putative transcription factors (Llave et al., Science, 297: 2053-2056, 2002). The Drosophila bantam gene encodes a miRNA that regulates cell proliferation and the pro-apoptotic gene hid (Brennecke et al., Cell, 113: 25-36, 2003). Evidence supporting the notion that miRNAs are an important class of regulatory molecule is growing.
Given the fundamental biological processes that are regulated by miRNAs and the knowledge that many of these processes are altered in a variety of human conditions, it is important to determine whether miRNAs play a role in these conditions. For example, miRNAs have recently been implicated in carcinogenesis and development and differentiation of numerous cell types.
Metzler et al (Gene Chromosomes Cancer, 39(2): 167-9, 2004) reported recently that mir-155/bic RNA expression is up-regulated significantly in children with Burkitts Lymphoma. Recent studies by Michael et al (Mol Cancer Res. 1(12), 882-91, 2003) has shown that specific miRNAs shown reduced accumulation in colorectal neoplasia. Calin et al (Proc Natl Acad Sci USA., 99(24):15524-9, 2002) found an association between chronic lymphocytic leukemia (CLL) and deletions in a region of chromosome 13, which contains the coding regions for the miRNAs miR-15 and miR-16. They found that these miRNAs are either absent, or down-regulated, in a majority of CLL specimens (˜68%). Hemizygous and/or homozygous loss at 13q14 constitute the most frequent chromosomal abnormality in CLL. Deletions at this region also occur in approximately 50% of mantle cell lymphomas, in 16-40% of multiple myelomas, and in 60% of prostate cancers, suggesting the involvement of one or more tumor suppressor genes at this locus. Although several groups have performed detailed genetic analysis, including extensive loss of heterozygosity (LOH) analysis, mutation, and expression studies, no consistent involvement of any of the genes located in the deleted region has been demonstrated. If loss of the 13q14 miRNA R-15 and R-16 locus is key for the genesis of CLL, then these data by Calin et al are consistent with the idea that a miRNA may act as a tumor suppressor.
It is also possible that cancer could result from translocations of oncogene into miRNA loci. One such potential example of this is the translocation of MYC into the miRNA mir-142 loci, which causes an aggressive B cell leukemia due to strong up-regulation of MYC expression (Gauwerky et al., Proc Natl Acad Sci USA 86, 8867-8871, 1989). The MYC gene translocated only 4 nucleotides downstream of the mir-142 3′ end, and is likely under control of the upstream miRNA promoter. Alignment of mouse and human mir-142 containing EST sequences indicates ˜20 nucleotide conserved sequence element downstream of the mir-142 hairpin, which is lost in the translocation (Lagos-Quintana et al., Curr. Biol. 12:735-739, 2002). It was suggested that the absence of this conserved downstream sequence element in the putative mir-142/MYC fusion prevented the recognition of the transcript as a miRNA precursor to be properly processed, and therefore may have caused accumulation of fusion transcripts and overexpression of MYC. Thus there are multiple avenues for miRNA involvement in disease states, such as cancer, and the identification of miRNAs will likely help us to understand the cooperation of miRNA mechanisms in the biochemical mechanisms underlying the disease states.
Sempere et al. (Genome Biol. 5(3):R13. Epub 2004 Feb. 16, 2004) recently reported the identification of a subset of brain-expressed miRNAs whose expression behaviour is conserved in both mouse and human differentiating neurons. This data suggests that these miRNAs play a role in normal mammalian neuronal development and/or function. Furthermore, Houbaviy (Dev Cell. 5(2):351-8, 2003) identified a group of miRNAs in undifferentiated and differentiated mouse embryonic stem cells, with some of the miRNAs being specifically restricted to stem cells. The repression of these embryonic-specific miRNAs is repressed when the embryonic stem cells beings to differentiate. This suggests a role for miRNAs in the maintenance of the pluripotent cell state and direction of early mammalian development.
Approximately 220 miRNAs have been identified in humans and many of the identified miRNAs have been associated with important biological functions. By bioinformatics approach, Bartel and Burge (2003) estimated that up to 1% of the human genome may code for miRNAs. The roles of miRNA played in normal tissue development and cellular functions are just beginning to be explored. However, discoveries in the ncRNA field are severely hindered by the lack of efficient analytical tools. The timely development of a powerful tool to aid the study of ncRNA, such as miRNA, molecules is therefore needed. The present disclosure provides such an analytical tool for the analysis of ncRNAs. The present disclosure provides methods describing the detection and analysis of miRNAs. However, the methods of the present disclosure may also be applied to other ncRNAs as would be obvious to one of ordinary skill in the art.